Dehydration Types: Pathophysiology, Lab Tests and Values

Types of Dehydration

Dehydration is a lack of water in the body or water deficiency. Dehydration can be categorized into isotonic, hypertonic and hypotonic, depending on how it affects the tonicity of the extracellular fluid 1. The extracellular fluid includes the blood and the fluid between the cells (interstitial fluid).

Tonicity is the ability of the solution on one side of the cell membrane to attract water from the solution on the other side of the membrane. Normally, the solutions inside and outside the cells have the same ability to attract water, so they are isotonic [from the Greek iso = equal]. The tonicity of the fluid depends on osmotically active solutes. The main osmotically active solute that determines the tonicity of the extracellular fluid is sodium.

What are osmosis and osmolality?

Osmosis is the movement of water through the membrane from a solution with lower tonicity to a solution with higher tonicity. Substances that increase the tonicity of solutions and thus osmosis are called osmotically active substances. Sodium is the main osmotically active substance in the extracellular fluid. In dehydration, the concentration of sodium in the extracellular fluid can change significantly. The measure of the amount of osmotically active substances in the solution is osmolality, which is expressed in milliosmoles of a solute per kilogram of water (mOsm/L). The normal range of the blood plasma osmolality is 285-295 mOsm/kg.

Why is it important to know if dehydration is iso-, hyper- or hypotonic? Because the type of dehydration can suggest its cause and because hypotonic dehydration must be treated with great caution to avoid severe neurological damage.

NOTE: Tonicity and osmolality are similar but not the same; for differences check here and here.

Isotonic (Isonatremic) Dehydration

Isotonic (isonatremic, iso-osmolar) dehydration occurs when proportionally the same amount of water and sodium is lost from the body, so the sodium concentration of the extracellular fluid and hence its tonicity do not change. Isotonic dehydration is the most common type of dehydration 20.

Possible causes of isotonic dehydration:

Hypertonic (Hypernatremic) Dehydration

Hypertonic (hypernatremic, hyperosmolar) dehydration occurs when proportionally more water than sodium is lost from the body. This results in an increased concentration of sodium in the extracellular fluid, which becomes hypertonic regarding the intracellular fluid and therefore attracts water from the body cells. This results in the cell shrinkage, which may include a significant shrinkage of the brain cells.

Hypotonic (Hyponatremic) Dehydration

Hypotonic (hyponatremic, hypoosmolar) dehydration occurs when proportionally more sodium than water is lost from the body. This results in a decreased concentration of the extracellular fluid, which becomes hypotonic in comparison to intracellular fluid, which attracts water from the extracellular fluid. This leads to cell swelling, possibly including the brain swelling (cerebral edema) 1.